Development of a Rat Head Exposure System for Simulating Human Exposure to RF Fields From Handheld Wireless Telephones C.K. Chou, 1 * K.W. Chan, 1† J.A. McDougall, 1 and A.W. Guy 2 1 Department of Radiation Research, City of Hope National Medical Center, Duarte, California 2 Wireless Technology Research, L.L.C., Washington DC The aim of this project was to develop an animal exposure system for the biological effect studies of radio frequency fields from handheld wireless telephones, with energy deposition in animal brains comparable to those in humans. The finite-difference time-domain (FDTD) method was initially used to compute specific absorption rate (SAR) in an ellipsoidal rat model exposed with various size loop antennas at different distances from the model. A 3  1 cm rectangular loop produced acceptable SAR patterns. A numerical rat model based on CT images was developed by curve-fitting Hounsfield Units of CT image pixels to tissue dielectric properties and densities. To design a loop for operating at high power levels, energy coupling and impedance matching were optimized using capacitively coupled feed lines embedded in a Teflon rod. Sprague Dawley rats were exposed with the 3  1 cm loop antennas, tuned to 837 or 1957 MHz for thermographically determined SAR distributions. Point SARs in brains of restrained rats were also determined thermometrically using fiberoptic probes. Calculated and measured SAR patterns and results from the various exposure configurations are in general agreement. The FDTD computed average brain SAR and ratio of head to whole body absorption were 23.8 W/kg/W and 62% at 837 MHz, and 22.6 W/kg/W and 89% at 1957 MHz. The average brain to whole body SAR ratio was 20 to 1 for both frequencies. At 837 MHz, the maximum measured SAR in the restrained rat brains was 51 W/kg/W in the cerebellum and 40 W/kg/W at the top of the cerebrum. An exposure system operating at 837 MHz is ready for in vivo biological effect studies of radio frequency fields from portable cellular telephones. Two-tenths of a watt input power to the loop antenna will produce 10 W/kg maximum SAR, and an estimated 4.8 W/kg average brain SAR in a 300 g medium size rat. Bioelectromagnetics 20:75–92, 1999. © 1999 Wiley-Liss, Inc. Key words: microwave; loop antenna; specific absorption rate; RF dosimetry INTRODUCTION In the past four decades, animal microwave biolog- ical effect studies have been mostly whole-body expo- sure, using horn antennas, cavities, waveguides, Craw- ford Cells, parallel plates, or striplines. A review of these exposure methods can be found in Chou [1998]. Several head exposure studies were conducted with aperture ap- plicators on cats [Johnson and Guy, 1972], and rats [Sanders and Joines, 1984]; waveguide applicators on rats [Lenox et al., 1976; Guy and Chou, 1982], and guinea pigs and cats [Chou et al., 1982] to study micro- wave effects on nervous systems. For microwave cata- ractogenesis studies, Kramar et al. [1975] used a dia- thermy system, 2450 MHz C-director applicator, to irra- diate rabbit and monkey eyes [1976]. Kues et al. [1985] also irradiated monkey eyes in the near field of a 2450 MHz aperture waveguide. The only animal study that involved partial brain exposure was a blood– brain barrier study re- ported by Neilly and Lin [1986], which irradiated only one side of a rat head with a 3.15 GHz dielectrically loaded coaxial applicator. Recent prevalent use of handheld wireless tele- phones and concerns about their safety have restimulated research in this area. Burkhardt et al. [1997] described a Contract grant sponsor: Wireless Technology Research, L.L.C. †Current address for Kwok Chan: FCC Laboratory, Columbia, MD 20146. *Correspondence to: Dr. C.K. Chou, Motorola Florida Laboratories, 8000 W. Sunrise Blvd., Plantation, FL 33322. E-mail: ecc017@email.mot.com. Received January 1998; Final revision received 26 August 1998 © 1999 Wiley-Liss, Inc. Bioelectromagnetics 20:75–92 (1999)